In a communication system, air interface transmission has time delay. In order to ensure that uplink signals sent by a user equipment (User Equipment, UE) under a base station may arrive at the base station synchronously, the UE needs to obtain a timing advance (Timing Advance, TA) value before accessing to a cell to transmit uplink data, so that when performing uplink transmission at the cell, the user equipment always sends the uplink signals ahead of the TA to enable the uplink data to arrive at the base station at an appointed time. With moving of the user equipment, the TA value corresponding to a serving base station of the user equipment will change. The base station uses a TA command to inform the user equipment to change the TA value. A change value of the TA is carried in the TA command, for enabling the user equipment to correct the TA after receiving the change value of the TA. Meanwhile, the user equipment will set a timing alignment timer (Timing Alignment Timer, TAT) for maintaining whether the TA value is available. In the case of TAT overtime, the user equipment considers that the TA value may be unreliable, and then the user equipment considers that uplink is unreliable and needs to acquire the TA value again.
In a long term evolution (Long Term Evolution, LTE) system, the user equipment obtains uplink synchronization, namely, obtains the above-mentioned TA value, through a random access procedure (Random Access Procedure). The random access procedure is divided into two types, random access based on competition and random access based on non-competition.
In order to overcome uplink asynchronization, during the random access procedure, the user equipment performs random access by using a random access code. In order to avoid interference generated by an asynchronous user equipment, a physical random access channel (Physical Random Access Channel, PRACH) bearing the random access code needs more guard time (Guard Time), thus resulting in large resource occupation of the physical random access channel.
For the synchronized user equipment, when an uplink resource is requested, if the above-mentioned random access procedure is stilled used, the access resource will be wasted.